[§ 512]. The use of the word that in expressions like I eat that I may live, &c., is a modification of the subjunctive construction, that is conveniently called potential. It denotes that one act is done for the sake of supplying the power or opportunity for the performance of another.
The most important point connected with the powers of that is the so-called succession of tenses.
[§ 513]. The succession of tenses.—Whenever the conjunction that expresses intention, and consequently connects two verbs, the second of which takes place after the first, the verbs in question must be in the same tense.
I do this that I may gain by it
I did this that I might gain by it.
In the Greek language this is expressed by a difference of mood; the subjunctive being the construction equivalent to may, the optative to might. The Latin idiom coincides with the English.
A little consideration will show that this rule is absolute. For a man to be doing one action (in present time) in order that some other action may follow it (in past time) is to reverse the order of cause and effect. To do anything in A.D. 1851, that something may result from it in 1850 is a contradiction; and so it is to say I do this that I might gain by it.
The reasons against the converse construction are nearly, if not equally cogent. To have done anything at any previous time in order that a present effect may follow, is, ipso facto, to convert a past act into a present one, or, to speak in the language of the grammarian, to convert
an aorist into a perfect. To say I did this that I may gain by it, is to make, by the very effect of the expression, either may equivalent to might, or did equivalent to have done.
I did this that I might gain.